Storage batteries having zinc (Zn) as negative electrodes include the zinc nickel battery, zinc silver battery, zinc air battery and zinc manganese dioxide battery. All these batteries share a common disadvantage of having a short cycle life. The product from Zn during discharge has a relatively high solubility in the alkaline electrolyte of these batteries. Therefore, during the charge and discharge process, Zn repeatedly dissolves in the electrolyte solution and precipitates out of the electrolyte onto the electrode. However, the precipitation does not occur at the same location on the electrode where the Zn had previously dissolved. Since the current density is not distributed uniformly on the electrode, the quantity of dissolved Zn at the edge of the electrode is greater than that of precipitation. At the center of the electrodes, this phenomenon is reversed. This results in the redistribution of zinc such that the active material congregates at the center of an electrode and causes the deformation or change in the shape of the zinc electrode. During the cycling process, this deformation or shape change gradually decreases the actual surface area and, therefore, reduces a battery's capacity and shortens its cycle life.
In order to limit the deformation or shape change of the zinc electrode, research has been conducted to limit the migration of zinc during the charge and discharge process or to decrease the solubility of the zinc product in the electrolyte during discharge. Other researches have tried to change the non-uniformity of current density distribution on the electrode.
Adding a polymer-binding agent to the zinc negative electrode is one way to decrease the migration of the product during discharge. Polytetrafluoroethylene (PTFE) is commonly used. PTFE changes a component of zinc negative electrodes and limits the migration of zinc product during discharge. Duffield A, Mitchell P J, Kumar N, et al., Rotating-disk Study on Teflon-Bonded Porous Zinc Electrodes, J Power Sources, 1985, 15: 93. In these zinc electrodes, PTFE forms a three-dimensional net structure that can be seen under a microscope when the materials other than the PTFE are removed. This type of three-dimensional net structure made by PTFE is widely used in air electrodes. When air electrodes are fabricated using the method of fabrication for zinc electrodes, most materials filling in electrode slices are not active materials but materials such as sodium sulfate that can be removed by dissolving it in water. After sodium sulfate is removed by dissolving it in water, its occupied space is empty such that the remaining three-dimensional net structure can be observed easily. The purpose of forming this three-dimensional net structure in a zinc electrode is not to form empty spaces, but to use the three-dimensional net structure to pack the active material, blocking the discharge product of zinc electrodes and making migration difficult. Due to the difficulty in migration, the soluble zinc product that is continuously produced during every discharge at the discharge location also precipitates nearby after super-saturation.
However, PTFE is a type of organic macromolecule material and cannot provide the crystal nucleus for the precipitation of zinc product during discharge. The soluble zinc product during discharge only precipitates on the surface of un-discharged zinc in the forms of ZnO or Zn(OH)2 after super-saturation. They do not precipitate on the PTFE. Therefore, even though the migration is limited by this organic macromolecule material, a portion of the zinc product during discharge still diffuses away due to the force created by the concentration gradient of the soluble zinc. Thus, PTFE cannot fully stop the shape change or deformation of the electrode.
A method to decrease the solubility of the zinc product during discharge is to add an additive, such as calcium hydroxide (Ca(OH)2), to the electrode so that the Zn product during discharge would be insoluble. As explained in U.S. Pat. No. 5,460,899, the fundamental principle behind this process is that Ca(OH)2 can react with the zinc product that is dissolved in the electrolyte solution during the discharge process in order to form calcium zincate [Ca(OH)2.2Zn(OH)2.2H2O] that is insoluble and causes the zinc product to precipitate during discharge. However, during the fabrication of the negative electrode, when the electrode paste containing the Ca(OH)2 is stirred during mixing, part of the Ca(OH)2 will react with the carbon dioxide in the air to form calcium carbonate causing it to lose its effectiveness. In order to avoid the reaction with carbon dioxide, the electrode paste will have to be stirred in a hermetically sealed environment. This will increase the equipment cost necessary for the production of the battery.
Another method to decrease the solubility of zinc product during discharge is to add some auxiliary salts such as K2CO3 or KF in an electrolyte containing KOH. As explained in U.S. Pat. No. 5,302,475, the fundamental principle behind this method is that the solubility of zinc product during discharge significantly decreases in an electrolyte containing KOH and a combination of KF and K2CO3 salts. However, in this three-electrolyte solution of KOH—KF—K2CO3 electrolyte, the solubility of KOH also significantly decreases. This results in the decrease of solution's alkalinity. Therefore, even though this electrolyte decreases the solubility of zinc product during discharge and inhibits the shape change or deformation of the zinc negative electrode, it also degrades the properties of the positive electrode of the battery such that it does not significantly extend the life of the battery.
Due to the limitations of the prior art, it is therefore desirable to have negative electrodes and novel methods of fabricating these negative electrodes such that the zinc electrodes do not lose their surface area during the cycling process and batteries with these electrodes have a higher capacity.